The present invention relates to a fuel injection system and a method for injecting hydrogen fuel, in particular liquid hydrocarbon fuel, into a fuel reformer for generating a hydrogen rich gas from said hydrocarbon fuel, wherein said hydrocarbon fuel is sprayed into the fuel reformer.
Hydrogen rich gas can be used as an intermediate product in order to produce eventually hydrogen. Hydrogen can be used for many different purposes. For instance, hydrogen is necessary for the operation of fuel cells which provide electric energy for a vehicle or other applications. Hydrogen can be directly stored in a tank, but this storage is technically complicated and dangerous due to the explosiveness of hydrogen. Therefore, it has proven useful to generate a hydrogen rich gas from (preferably liquid) hydrocarbon fuel through catalytic conversion.
The catalytic conversion of the hydrocarbon chains contained in the hydrocarbon fuel is well-known and is therefore only summarized in the following. Usually, the conversion is carried out in several successive steps, comprising the step of the actual reforming, in which the hydrocarbon chains contained in the liquid hydrocarbon fuel are broken down and eventually converted into a hydrogen rich gas comprising hydrogen, carbon monoxide, carbon dioxide, steam and usually also to a certain extent remaining hydrocarbon chains in accordance with the thermodynamic equilibrium.
The hydrogen rich gas is then further processed in subsequent stages in a well-known manner in order to produce hydrogen in the required degree of purity, for instance by applying subsequent staged shift reactions, during which carbon monoxide and water are catalytically converted to carbon dioxide and hydrogen, and, if necessary, by applying subsequent cleaning processes in which other unwanted substances in the hydrogen rich gas (as for instance the remaining hydrocarbon chains) are removed or converted into chemical compounds that do not harm the use of the hydrogen eventually produced at the end of the total hydrogen production process.
For the first step in this process, namely the catalytic conversion of hydrocarbon fuel to a hydrogen rich gas, a so called fuel reformer is used. For good operating modes of the fuel reformer it has been shown that a successful and efficient conversion of a hydrocarbon fuel into a hydrogen rich gas is, among others, dependent on a successful mixing of the reactants. For that mixing it has proven useful to spray the hydrocarbon fuel into the fuel reformer, whereby a good atomization or vaporization of the hydrocarbon fuel can be achieved.
Disadvantageously, when liquid hydrocarbon fuel is used in a fuel reformer, it is difficult to regulate the amount of injected hydrocarbon fuel and at the same time keep a high quality of the spray regarding degree of atomization or vaporization and of mixture with other reactants like water or steam.
In practice, a fuel reformer needs to be operated in a wide range of fuel flow rates, e.g. between 2.5 g-25 g fuel per minute. Usually, the operational range of the fuel reformer is expressed in terms of a fixed ratio of the minimum flow rate (in the example above: 2.5 g fuel per minute) to the maximum flow rate (in the example above: 25 g fuel per minute). This ratio is called the “turn down ratio”, whereby in the example above the turn down ratio is 1:10. In general, a high turn-down ratio is desired, which means at the same time that the fuel reformer can be operated in a wide range of fuel flow rates.
However, in practice it is rather difficult to design a fuel injection element which produces an optimal spray or atomization of the fuel over a wide range of fuel flow rates. Usually, the geometry of the fuel injection element is designed for one fuel flow rate, e.g. either the above mentioned minimum flow rate of 2.5 g per minute or the above mentioned maximum fuel flow rate of 25 g per minute. Thereby, the atomization of the fuel spray can be influenced by a plurality of different parts of the fuel injection element. For an easier understanding, the design problem will be described using the example of the size of the fuel spray outlet hole.
For example the size of the fuel spray outlet hole can be either optimized for the minimum flow or the maximum flow. In case the size of the hole is designed for achieving a good atomization of the fuel spray for a low fuel amount (small hole), e.g. 2.5 g per minute, the size of the hole will be rather small and it will be very difficult to force a higher fuel amount, e.g. 25 g per minute through such small hole, without increasing the pressure of the fuel before the hole excessively. On the other hand, if the size of the hole is designed for achieving a good atomization of the fuel spray for a high fuel amount, e.g. 25 g per minute, (large hole), the size of the hole will be rather large and the use of a low fuel amount, e.g. 2.5 g per minute, will not result in an sufficiently atomized fuel spray, but rather in a dribbling of the fuel, as the pressure of the fuel at the hole is too low for the production of an atomized fuel spray. Consequently, to design a nozzle producing a good spray quality for both high and low flow rates using only one standard nozzle design is a problem.
Nevertheless a wide operation range or an increased turn-down ratio, respectively, of the fuel reformer is desired.
For increasing the turn-down ratio, one possibility is to increase the amount of fuel sprayed into the fuel reformer. In general, an increase of the amount of fuel can be achieved by increasing the pressure of hydrocarbon fuel supply, whereby more hydrocarbon fuel is forced to pass through the fuel injection element. This has the disadvantage that the fuel injection element, but also the fuel supply lines, the valves and all other involved parts for supplying fuel to the fuel injection element need to be designed for high pressure applications. Additionally, also the pump in the fuel supply system providing the wanted fuel pressure has to be designed for the necessary high pressure of the fuel. All these factors increase the overall costs of the system.
As already mentioned above, the other possibility to increase the turn down ratio is to design the whole fuel injection element for a higher fuel amount. However, this has the disadvantage that for a low or the minimum fuel flow rate, the degree of vaporization or atomization of the fuel spray in the fuel reformer deteriorates, so that a good mixing of the reactants cannot be provided in the fuel reformer.
It is therefore desirable to provide a cost efficient fuel injection system and method which provides an increased quality of the spray, but also provides an increased turn down ratio.
An aspect of the present invention is based on the idea to provide (i) a fuel injection system for injecting (preferably liquid) hydrocarbon fuel into a fuel reformer, which is adapted to inject a pulsating spray of hydrocarbon fuel into the fuel reformer, and (ii) a method for injecting liquid hydrocarbon fuel into a fuel reformer, said method comprising the step of injecting the hydrocarbon fuel as a pulsating spray of hydrocarbon fuel into the fuel reformer. Preferably, the hydrocarbon fuel is injected by at least one fuel injection element into the fuel reformer.
In a preferred embodiment of an aspect of the invention, the fuel injection system comprises a valve with at least one output port and at least one input port, wherein the at least one output port of the valve is connected to at least one fuel injecting element and the at least one input port of the valve is adapted to receive hydrocarbon fuel from a fuel supply system and wherein the valve is adapted to provide a (liquid or gaseous) hydrocarbon fuel stream having a pulsating pressure.
Preferably, the fuel injection system further comprises a fuel stream stopping element, preferably a check valve, which is adapted to stop the supply of the hydrocarbon fuel stream to the at least one fuel injection element, if the pressure of the fuel stream supplied at the at least one fuel injection element falls below a predetermined threshold value. Otherwise, i.e. if the pressure of the fuel stream supplied at the at least one fuel injection element exceeds (or is at least equal to) the predetermined threshold value, it provides the liquid hydrocarbon fuel stream having the wanted pulsating pressure. Alternatively, the supply of fuel can already be stopped when the pressure of the fuel stream supplied is equal to the predetermined threshold value.
By providing an immediate cut off of the pressure of the fuel at the at least one fuel injection element in case the pressure of the fuel falls below a predetermined value, e.g. below 1.5 bar, the fuel spraying process inside the fuel reformer stops more or less immediately, too. This means that the transition period from (i) fuel spraying in the fuel reformer under full fuel pressure before the cut off to (ii) the stop of the fuel spraying after the cut off is considerably shorter when using such fuel stream stopping element than without using it. The shortening of the transition period by the use of such a fuel stream stopping element in turn reduces considerably the negative impact the decreasing pressure of the fuel at the at least one injection element may have onto the quality of the fuel spray during this transition period.
Preferably, the fuel stream stopping element is arranged near the at least one fuel injection element, e.g. between the output port of the valve and the at least one fuel injection element, and most advantageously it is arranged as near as possible to the fuel spray outlet of the at least one fuel injection element. Thereby, only a rather small amount of fuel, which might produce a fuel spray of lower quality in the fuel reformer due to the decreasing fuel supply pressure, is contained in the space between the fuel stream stopping element and the fuel spray outlet of the at least one fuel injection element after the fuel stream supply has been cut off, i.e. has been stopped. Consequently, the quality of the fuel spray in the fuel reformer is still rather good, even if the fuel pressure is nominally too low for achieving a good atomization of the fuel spray.
Advantageously, the fuel stream stopping element and the at least one fuel injection element can be co-designed to form one or more integral devices, for instance a fuel injection nozzle with a built-in fuel check valve. Such fuel injection nozzles with a built-in fuel spray stopping elements are known from the state of the art (for instance the LE-series nozzles provided by Danfoss or the drip-free misting nozzle provided by Steinen, see e.g. the general data sheet for Oil Nozzles Type LE by Danfoss available on the internet at http://no.varme.danfoss.com/PCMPDF/DKBDPDQ6QD302.pdf and the information on Drip Free Misting Nozzles by Steinen also available on the internet at http://www.steinen.com/pdf/DripFree.pdf).
Further, it is also possible to co-design the valve and the fuel stream stopping element to form a single integral device. Thereby, the amount of fuel remaining in the space between the fuel stream stopping element and the at least one fuel injection element may be larger than in case where the fuel stream stopping element is arranged near or integral with such fuel injection element. But, since the necessary overall constructional space of the whole fuel injection system will be decreased in most of the cases any increase in the amount of fuel remaining in the space between the fuel stream stopping element and the at least one fuel injection element will be relatively small and can therefore usually be neglected.
In principal it is also possible and encompassed by the scope of the claimed invention, to integrate the valve, the fuel stream stopping element and the fuel injection element in one single integral device. In this case, preferably the valve is adapted to operate at high temperatures, since the fuel injection element will be arranged close to the mixing chamber of the fuel reformer which operates at high temperatures (e.g. above 400 0C). Since such valves are usually designed as magnetic valves which are electrically operated, such known standard valves can not operate properly at higher temperatures, e.g. above 100 0C. Therefore, in case such a single integrated device is used, either the system is adapted (i) to provide a cooling for the valve integrated into the device or (ii) to use a special valve designed for high temperature operation.
Further, the fuel injection system may comprise a fuel supply system comprising a fuel tank for containing the (liquid or gaseous) hydrocarbon fuel that is connected via a fuel supply line with the valve, and a pump for pressurizing the hydrocarbon fuel contained in the fuel supply line, wherein the fuel is kept in the fuel tank at a substantially constant or only slightly varying pressure level, preferably at a pressure level substantially equal to the actual atmospheric pressure the fuel injection system is exposed to.
Still further, the fuel injection system may comprise a reduction valve, preferably of the back pressure type, which is in connection with the fuel tank and the fuel supply line, thereby providing a back flow possibility for the fuel into the fuel tank in order to prevent the fuel in the fuel supply line from developing an overpressure and/or to keep the fuel in the fuel supply line at a substantially constant or only slightly varying pressure level, which corresponds to the operating pressure of the system.
Still further, the valve in the fuel injection system according to an aspect of the invention may advantageously be a 3-way valve with three ports, namely a fuel supply port, a pressure relief port and a fuel injection element connection port, wherein the fuel supply port and the pressure relief port are in connection with the fuel tank via the—pressurized—fuel supply line and via a pressure relief line, respectively. The fuel injection element connection port is—via a fuel injection element connection line—in connection with the fuel stream stopping element and subsequently with the at least one fuel injection element or directly with the at least one fuel injection element.
Alternatively, in case of more than one fuel injection element instead of having one central fuel stream stopping element for all subsequent fuel injection elements there could be more such fuel stream stopping elements, each fuel injection element being operably connected to its “own” preceding fuel stream stopping element.
The 3-way valve is preferably designed and operated in such a way that it—in its non-activated state—is closed to the pressurized fuel supply line (closed fuel supply port), while its pressure relief port is open and operably connected via the pressure relief line with the fuel tank. The fuel injection element connection port serves as input port, whereby, in case there is a certain pressure remaining at the fuel stream stopping element and/or the at least one fuel injection element respectively after the closing of the fuel supply port of the valve, the pressure in the at least one fuel injection element connection line and/or the valve can be further reduced by providing a back flow possibility of the fuel at the at least one fuel injection element respectively the fuel stream stopping element into the fuel tank.
In its activated state, the valve is closed to the pressure relief line (closed pressure relief port), and its fuel supply port is operably connected to the pressurized fuel supply line. The fuel injection element connection port serves as output port, whereby pressurized fuel is provided in the fuel injection element connection line, and subsequently at the fuel stream stopping element and/or at the at least one fuel injection element.
Preferably, the 3-way valve is electrically actuated. By temporarily activating the 3-way valve, the pressure relief line of the fuel supply system is closed and the pressurized fuel supply line of said system is opened, whereby fuel having a high pressure is supplied to the at least one fuel injection element. As soon as the pressurized fuel supply line is closed again, the pressure of the fuel is reduced to a lower pressure level, preferably substantially the actual atmospheric pressure the system is exposed to.
Preferably, the pressure of the fuel in the pressurized fuel supply line is higher than the threshold pressure of the fuel stream stopping element, so that the fuel stream stopping element is opened and providing a fuel stream of pressurized fuel at the fuel injection element once the valve is activated. As soon as the supply of pressurized fuel is terminated by closing the pressurized fuel supply line of the valve, the remaining pressure in the fuel injection element connection line is reduced by the back flow of fuel through the open fuel injection element connection port and the opened pressure relief port of the valve into the fuel tank. Thereby, a fuel supply at the at least one fuel injection element is abruptly stopped upon reaching the threshold pressure of the fuel stream stopping element by immediately closing said fuel stream stopping element. Such fuel stream stopping element could be a known check valve.
Since providing a hydrocarbon fuel stream with pulsating pressure also causes undesired fluctuations in the fuel supply lines which in turn can damage the pump and the 3-way valve and should therefore be reduced, a gas accumulator can be advantageously fitted to the fuel supply lines, preferably close to the 3-way valve. The gas accumulator serves as buffer to compensate the variations of the pressure in the fuel supply lines caused by the pulsating release of fuel injections.
According to a preferred embodiment, the 3-way valve is operated with a frequency that depends on how fast the fuel stream stopping element can stop the fuel supply. This in turn mainly depends on how fast a pressure relief through the pressure relief line can be provided. The pressure relief in turn depends on how fast the 3-way valve can be operated. Practice shows that good results can be achieved when operating the fuel supply system at rather low frequencies, thereby providing at the at least one fuel injection element (for instance an injection nozzle) a hydrocarbon fuel supply stream which pressure varies and pulsates at rather low frequencies. This pulsating hydrocarbon fuel supply stream in turn provides a correspondingly pulsating hydrocarbon fuel spray in the fuel reformer of sufficiently high quality. In addition, it allows also increasing the amount of fuel sprayed into the fuel reformer per time unit. In a preferred embodiment of the invention, the fuel stream stopping element, for instance the check valve, is operated with a frequency of less than circa 50 Hertz, preferably less than circa 20 Hertz.
Since the fuel is injected only temporarily at higher pressure values, usually the fuel injection system does not need to be designed for high pressure applications. Additionally, because of such temporarily increased pressure values the operation range of the fuel injection element can be increased by providing a possibility to operate the fuel injection system with a low fuel amount and at the same time design the fuel injection element for a larger amount of fuel that can be injected into the fuel reformer per time unit. Since the fuel spray is intermittently pulsating, the atomization or vaporization of the spray in the fuel reformer is still good. In this context, it should be noted that the mixing zone of the fuel reformer can be regarded as low pass filter which levels out the pulsations in the atomized fuel spray.
The average fuel pressure is preferably in the range of circa 8 to circa 15 bar . above atmospheric pressure, and in particular in the range of circa 10 to circa 12 bar above atmospheric pressure, whereas the pressure in the fuel reformer is usually less than circa 4 bar above atmospheric pressure, and preferably less than circa 2 bar above atmospheric pressure.
Further advantages and preferred embodiments are defined in the claims, the figure and the description.